U.S. patent number 5,132,728 [Application Number 07/521,524] was granted by the patent office on 1992-07-21 for drum driving unit of electrophotography printer.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Kenichi Ebata, Yoshifumi Homma, Tsugio Kikuchi, Yousuke Nagano, Masafumi Suzaki.
United States Patent |
5,132,728 |
Suzaki , et al. |
July 21, 1992 |
Drum driving unit of electrophotography printer
Abstract
An electrophotography printer includes a drum formed of a
non-magnetic cylindrical body having an outer circumferential
surface to which a photosensitive material is applied, end brackets
attached to opposite ends of the drum, and a drum shaft for
transmitting a driving torque to the drum. A connection structure
between the drum and the drum shaft includes end bracket shafts
provided on the end brackets, a hole formed in the drum shaft so as
to be capable of engaging with one of the end bracket shafts,
parallel pins provided on the end bracket shafts so as to be
perpendicular to the axes thereof, and at least one taper groove
formed in an end portion of the drum shaft. When the drum is
housed, one of the parallel pins and the taper groove engage with
each other while the whole of the drum is pressed toward the drum
shaft by an elastic member, thereby reducing the non-uniformity of
the rotation of the drum.
Inventors: |
Suzaki; Masafumi (Hitachi,
JP), Kikuchi; Tsugio (Ibaraki, JP), Nagano;
Yousuke (Hitachi, JP), Homma; Yoshifumi (Hitachi,
JP), Ebata; Kenichi (Hitachi, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
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Family
ID: |
14882645 |
Appl.
No.: |
07/521,524 |
Filed: |
May 10, 1990 |
Foreign Application Priority Data
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May 19, 1989 [JP] |
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1-124329 |
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Current U.S.
Class: |
399/167; 403/355;
492/15 |
Current CPC
Class: |
G03G
15/757 (20130101); Y10T 403/7018 (20150115) |
Current International
Class: |
G03G
15/00 (20060101); G03G 015/00 () |
Field of
Search: |
;355/200,210,211,212
;403/354,355,378,379 ;29/115,116.1,122,123 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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193931 |
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Nov 1983 |
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JP |
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96317 |
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Apr 1988 |
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JP |
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75274 |
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May 1989 |
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JP |
|
Primary Examiner: Moses; R. L.
Assistant Examiner: Horgan; Christopher
Attorney, Agent or Firm: Antonelli, Terry Stout &
Kraus
Claims
What is claimed is:
1. A drum driving unit of an electrophotography printer including a
drum formed of a non-magnetic cylindrical body having an outer
circumferential surface to which a photosensitive material is
applied, end brackets attached to opposite ends of said drum, and a
drum shaft for transmitting a driving torque to said drum, said
drum driving unit comprising:
end bracket shafts provided on said end brackets;
a hole formed in said drum shaft, said hole being capable of
engaging with one of said end bracket shafts;
parallel pins provided on said end bracket shafts so as to be
perpendicular to the axes thereof; and
at least one tapered groove formed in an end portion of said drum
shaft;
wherein when said drum is housed, one of said parallel pins and
said taper groove engage with each other while the whole of said
drum is pressed toward said drum shaft by an elastic member,
and
wherein an angle .theta. of said taper groove is selected so that
if an elasticity force applied to said drum is W.sub.s, the driving
load torque of said drum is T and the average diameter of said drum
shaft is D, 2T/D.multidot.tan.theta.<W.sub.s can be established,
and the elasticity force W.sub.s ranges to at most 3 kg.
Description
BACKGROUND OF THE INVENTION
This invention relates to an electrophotography printer such as a
laser beam printer and, more particularly, to a drum driving unit
of this kind of printer.
A drum driving unit of a conventional electrophotography printer
will be described below with reference to FIG. 2. An end bracket 2
is attached to an end of a drum 1 constituted by a non-magnetic
cylindrical body having an outer circumferential surface to which a
photosensitive material is applied. An end bracket shaft 3 is
attached to the end bracket 2 coaxially with the same. A spring
type one-way clutch 5 is coaxially incorporated in a drum shaft 4
for transmitting a driving torque to the drum 1. The drum 1 is an
expendable member and needs to be changed periodically. To change
the drum 1, a drum assembly 6 integrally formed the drum 1, the end
bracket 2 and the end bracket shaft 3 is changed. The end bracket
shaft 3 of a new drum 1 is brought into engagement with the one-way
clutch 5. In ordinary electrophotography printers, the drum 1 is
rotated in only one direction. The arrangement may therefore be
such that the one-way clutch 5 is locked with respect to the
direction of rotation of the drum 1 to enable the torque of the
drum shaft 4 to be transmitted to the drum assembly 6. At the time
of of the exchange, the attachment position of the drum 1 in the
rotational direction is not particularly limited, and it is
sufficient to establish engagement between the end bracket shaft 3
and the one-way clutch 5 incorporated in the drum shaft 4.
The above-described conventional art entails a problem described
below.
FIG. 3 shows the results of an actual test which was conducted in
such a manner that an optical encoder was attached to the drum of a
conventional type of printer to detect the rotational speed of the
drum and, hence, to measure the variation in the rotational speed
(unevenness) with respect to a reference speed during one
revolution of the drum. In FIG. 3, segments B which indicate
abnormally large speed variations are clearly seen between segments
A which indicate speeds in a normal range. Variations in the drum
speed are caused by the variation in the force of friction between
the drum and a blade cleaner for removing surplus part of the ink
applied to the drum, but deteriorations of the image qualities
caused by the influence of these variations are small. Therefore
the present invention is not intended to remove small speed
variations such as those indicated by the segments A. FIG. 4
schematically shows features of an actual image sample printed on
the condition shown in FIG. 3. This actual image sample is called a
half tone image formed by drawing lateral lines with small regular
pitches to obtain a half tone. Since the recent market trend is
being shifted from setting importance on characters to setting
importance on graphics, the subject of how to output half tone
images having improved qualities is important. In the conventional
printers, however, the printing pitches in the direction of
rotation of the drum is made uneven due to unevenness of the
rotation of the drum 1 (speed variations), resulting in the
formation of a light-shade striped pattern, such as that shown in
FIG. 4. This phenomenon considerably deteriorates the image
qualities. It is therefore very important to prevent this
phenomenon.
The reason for the occurrence of such large variations in the
rotational speed is that the one-way clutch is repeatedly locked
and released irregularly by small vibrations and variations in the
load, since the one-way clutch has a restraining force with respect
to rotation in only one direction and has no restraining force in
the opposite direction.
Japanese Utility Model Unexamined Publication No. 1-75274 discloses
a construction in which the entire drum is attached with a pressing
force of a spring. However, it is not possible to attain an object
of the present invention, i.e., the object of preventing unevenness
of the drum rotation and, hence, light-shade unevenness of a half
tone by only simply pressing the drum.
Japanese Patent Unexamined Publication No. 58-193931 discloses a
method relating to a shaft joint, in which a member constituted by
an elastic body having a taper groove is attached to a groove
portion of a shaft, and this groove portion is engaged with a
spring pin. For driving a laser printer drum, however, it is not
suitable to apply the construction in which an elastic member is
used for a shaft joint, because the elastic member is elastically
deformed with changes in the load to cause vibrations in the axial
direction, resulting in light-shade unevenness in the printed
image.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
drum driving unit for use in an electrophotography printer in which
a structure including the connection between a drum which is an
expendable member and which needs to be changed and a drum shaft
for driving torque transmission which must therefore be separated
from the drum is optimized to reduce the non-uniformity of drum
rotation and, hence, to obtain good graphics while enabling the
drum to be changed in a simple manner.
To achieve this object, according to the present invention, a drum
driving unit is provided including a hole formed in the drum shaft,
with end bracket shafts, being formed on the end brackets so as to
be capable of engaging with the hole, and with parallel pins being
provided on the end bracket shafts so as to be perpendicular to the
axes thereof. A taper groove is formed in an end portion of the
drum shaft, and when the drum is housed, one of the parallel pins
and the taper groove engage with each other while the entire drum
is pressed toward the drum shaft by an elastic member.
According to further features of the present invention, a drum
driving unit is provided including a hole formed in at least one of
the end brackets, with an engagement shaft portion being formed on
the drum shaft so as to be capable of engaging with the hole, and
with a parallel pin being provided on the engagement shaft portion
perpendicularly to the same. A taper groove is formed in an end
portion of the end bracket where the hole is formed, and when the
drum is housed, the parallel pin and the taper groove engage with
each other while the whole of the drum is pressed toward the drum
shaft by an elastic member.
In the construction of the present invention including the hole in
the drum shaft and the end bracket shaft provided on the
corresponding end bracket, the axes of the drum shaft and the end
bracket shaft are aligned with each other and one end of the drum
assembly is supported by the drum shaft. The construction including
the parallel pins perpendicular to the end bracket shafts and the
taper groove formed in the end portion of the drum shaft, in which
the parallel pin and the taper groove engage with each other when
the drum is housed ensures that a driving torque of the drum shaft
is transmitted to the drum assembly. The construction in which the
entire drum is pressed toward the drum shaft by an elastic member
such as a spring ensures that the parallel pin is pressed against
taper surfaces of the taper groove so that the engagement between
the drum shaft and the end bracket shaft is free from any play.
Consequently, the drum shaft and the drum are connected like an
integral member, thereby eliminating the possibility of occurrence
of large variations in the rotational speed of the drum (unevenness
of rotation). Even if, when the drum is set in the printer, the
parallel pin is positioned on the drum shaft end without being
engaged with the taper groove, depending upon its position in the
direction of rotation of the drum, the pin can be easily brought
into engagement with the taper groove as the drum shaft rotates
during a warm-up of the printer, because a thrust force based on
the force of the elastic member, i.e. spring, is applied to the
entire drum.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of drum engagement portions in
accordance with an embodiment of the present invention;
FIG. 2 is a perspective view of drum engagement portions of a
conventional drum driving unit;
FIG. 3 is a graph showing data on the measurement of the variation
in the rotational speed of the conventional drum;
FIG. 4 is a schematic diagram of an actual image sample obtained by
a printer using the conventional drum driving unit;
FIG. 5 is a cross-sectional view of the construction of a laser
beam printer to which the present invention is applied;
FIG. 6 is a cross-sectional view of a unit for rotating the drum in
accordance with the present invention;
FIG. 7 is a graph showing data on the measurement of the variation
in the rotational speed of the drum in accordance with the present
invention;
FIG. 8 is a schematic diagram of an actual image sample obtained by
the printer using the drum driving unit of the present
invention;
FIG. 9 is a perspective view of an example of a modification of the
drum engagement portions in accordance with the present invention;
and
FIG. 10 is a diagram of the setting of the angle of the taper
groove portions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will be described below with
reference to FIGS. 1, 5, and 6. As shown in FIG. 5, a laser beam
printer in accordance with the present invention includes an
electrifier 7, an optical unit 8, a development device 9, a
transfer/discharge device 10, a cleaning unit 11 and an erasing
lamp 12 disposed around a drum 6 to which a photosensitive material
is applied. These components are arranged to effect an
electrophotography process. A printing sheet cassette 13, a
fixation unit 14 and a printing sheet discharge tray 15 are also
provided.
As shown in FIG. 6, a drum driving mechanism in accordance with the
present invention includes a drum assembly 18 disposed between a
pair of body side plates 16 and 17. The drum assembly 18 has a drum
19 in the form of a hollow cylinder formed of a non-magnetic
material such as aluminum and having an outer circumferential
surface to which a photosensitive material is applied, end brackets
20 respectively fixed by press-fitting to opposite ends of the drum
19, end bracket shafts 21 respectively fixed by press-fitting to
the end brackets 20, and parallel pins 22 respectively fitted by
press-fitting to the end bracket shafts 21. The drum assembly 18 is
symmetrical with respect to a plane transversely bisecting the
drum.
One of the end bracket shafts 21 is engaged with and is supported
on a drum shaft 23 for transmitting a driving torque to the drum
assembly 18 in a manner described more fully hereinbelow with
reference to FIG. 1.
The other end bracket shaft 21 is supported on the printer body by
a drum support plate assembly 29 consisting of a drum support plate
24, a bracket 26 and a ball bearing 28. The drum support plate 24
is detachably attached to the body side plate 17, the bracket 26 is
attached to the drum support plate 24 with screws 25, and the
bearing 28 is fitted in the bracket 26 so as to be movable in the
axial direction. The drum assembly 18 can be interchanged in such a
manner that the entire drum support plate assembly 29 is detached
and the drum assembly 18 is thereafter drawn out in the direction
of the arrow A.
A cylindrical coil spring 30 is provided in the bracket 26 to
constantly urge by the spring force the entire drum assembly 18 in
the direction of the arrow B through the ball bearing 28 and the
parallel pin 22.
A construction for transmitting the driving torque of the drum
driving motor to the drum shaft 23 will be described below.
A driving motor unit 44 has an opposed flat plate type dc motor. A
magnet 46 in the form of a disk is mounted on a flat plate 45. The
flat plate 45 is rotated by energizing a coil 47 disposed so as to
face the magnet 46. The driving torque of this plate is transmitted
to the drum shaft through a motor pinion 48 integrally attached to
the flat plate 45, a first stage gear 49, a second stage pinion 50,
a second stage gear 51, a first pulley 52, a timing belt 53, and a
second pulley 54.
As shown in FIG. 1, the end brackets 20, the end bracket shafts 21
and the parallel pins 22 are integrally attached to the opposite
ends of the drum 19 so as to be symmetrical with respect to a plane
transversely bisecting the drum 19. The drum shaft 23 has a hole
23a in which the end bracket shaft 21 is inserted (fitted). The
hole 23a is formed with a slight tolerance defined between its
diameter and the outside diameter of the end bracket shaft 21 so as
to be capable of being fitted around the same without any
substantial play in the radial direction at the time of attachment
of the drum as well as to be capable of being disengaged from the
end bracket shaft. Taper grooves 23b are formed in the shaft 23 at
an end of the same so as to be capable of receiving the parallel
pin 22 when the end bracket shaft 21 is inserted into the hole 23a.
The groove 23b are provided with taper surfaces in order to prevent
engagement play between the parallel pin 22 and the grooves and to
thereby prevent variations in the rotational speed (unevenness of
rotation) of the drum during rotation of the same. That is, since
the entire drum 18 is urged in the direction of the arrow B, the
outer circumferential surfaces of the parallel pin 22 are brought
into engagement with the taper portions of the taper grooves 23b
without permitting any engagement play. Even if, at the time of
setting of the drum 18, the parallel pin 22 does not engage with
the taper grooves 23b and is positioned on a drum shaft end 23c,
the cylindrical coil spring 30 is compressed to enable the drum
assembly 18 to be housed in the printer. Thereafter, as the drum
shaft 23 is rotated during warm-up of the printer (while the drum
is stopped at the set position), the parallel pin is moved to the
position at which it can be engaged with the grooves, thereby being
automatically brought into engagement. Additional taper grooves may
be provided so that the grooves are arranged crisscross, and
further taper grooves may be provided.
FIG. 7 shows the results of a measurement of the variation in the
rotational speed of a printer drum actually constructed in
accordance with the present invention. (FIG. 7 corresponds to FIG.
3 showing the speed variation in the conventional arrangement). As
is apparent from FIG. 7, the present invention is free from any
substantially large irregular speed variations, and the performance
is good.
FIG. 8 schematically shows the results of printing of a half tone
image actually performed by using the printer in accordance with
the present invention. As is readily apparent from FIG. 8, no
light-shape striped pattern is exhibited and a high-quality image
is obtained.
In the embodiment of FIG. 9, a fitting shaft portion 35a is
provided on a drum shaft 35, and a parallel pin 22 is press-fitted
in the fitting shaft portion 35a perpendicularly to the same. An
end bracket 20 is press-fitted in the drum 19. A boss 36 is
integrally formed on the end bracket 20. The boss portion 36 has a
fitting hole 36a in which the fitting shaft 35a of the drum shaft
is fitted, and taper groove portions 36b. This arrangement also
ensures the same effects described hereinabove in connection with
the embodiment of FIGS. 1, 5 and 6.
The setting of the angle of the taper grooves will be described
below with reference to FIG. 10. If the force of the cylindrical
coil spring for urging the drum is W.sub.s, W.sub.s must be
substantially smaller than an ordinary manual pressing force, i.e.,
3 kg or less. Assuming the load torque applied to the drum be T and
the average diameter of the drum shaft (the mean of outside and
inside diameters) be D as shown in FIG. 10, a force of W=2T/D is
applied to the parallel pin in a perpendicular direction.
The parallel pin is engaged with the groove portion having an
inclination of an angle .theta.. A thrust force therefore acts to
press the spring backward, which force is expressed by F.sub.s
=Wtan.theta.=2T/D.multidot.tan.theta.. F.sub.s must be smaller than
W.sub.s in order that the parallel pin is firmly engaged with the
taper groove portions. Accordingly, W.sub.s
.gtoreq.2T/D.multidot.tan.theta.must be established. In this
embodiment, T=6 kg-cm, D=2.2 cm and .theta.=15.degree. are set.
In accordance with the present invention, as described above, the
engagement between the drum shaft and the drum is free from any
play, and the non-uniformity of the drum rotation is reduced. It is
thereby possible to prevent occurrence of light-shade unevenness in
half tone images experienced as a problem of electrophotogaphy,
thus achieving an improvement in image qualities. The present
invention also makes it possible to realize a printer capable of
interchanging the drum by a simple operation and thus improved in
terms of maintenance facility.
Since the end brackets, the end bracket shafts and parallel pins
can be designed for common use in a symmetrical arrangement, the
drum assembly can be produced at a high efficiency, and the
possibility of occurrence of errors in assembly during production
or maintenance operation is reduced, thereby achieving improvements
in producibility and assembly facility.
The provision of the parallel pins at the opposite ends of the drum
makes it possible to simultaneously realize three functions of
transmitting a torque, preventing play and applying a thrust force.
It is therefore possible to provide a drum driving unit having a
simple construction and improve in performance.
The bearing disposed opposite to the drum shaft can have both
functions of supporting the shaft and applying a thrust force,
thereby being simplified in structure. Also, the
disassembly/assembly performance can be improved while the desired
low speed rotation precision is maintained, if the taper groove
angle .theta. is selected suitably. Since the taper grooves are
formed in a rigid body, there is no influence of torsional
vibrations in the rotational direction due to changes in the load,
which problem is encountered in the case of an elastic grooved
member. Stable rotational speed characteristics can therefore be
obtained.
* * * * *